The electric power industry has always been looking for economical devices that would effectively meter current usage. As long ago as 1881, Edison U.S. Pat. No. 215,545 disclosed an electric meter based on the known fact that a certain unit of current will cause the deposition of a definite amount of copper from a proper solution. Electric current usage was determined by weighing the copper of an electrolytic cell at the beginning and the end of the measuring period. Similar electro-plating coulometric cells have been employed for the same purpose in many subsequent patents. For example, Schweitzer U.S. Pat. No. 1,422,484 connected an electro-plating coulometric cell in series with an alternating current circuit to open the circuit upon passage of a predetermined quantity of current.
A versatile electro-plating coulometric cell is sold by Spraque Electric Company under the trademark Koolometer. It employs a gold wire for one electrode and a silver can for the other. Current usage is measured by deplating a known mass of silver which had been deposited on the gold wire. As the silver is removed from the gold wire, there is an almost imperceptible rise in the voltage drop across the cell, but when the last trace of silver has been deplated, the voltage rises sharply from one to three orders of magnitude. This voltage rise can be used as in Schweitzer U.S. Pat. No. 1,422,484 to interrupt the circuit. Of course, this would be impractical for normal electric metering use where the abrupt interruption of current would not be tolerated.
Regardless of the long-time availability of various electro-chemical coulometric cells, the electric power industry continues to meter current usage with electro-mechanical meters which are not only expensive but require periodic visual readings by a meter-reader. Present efforts at economy seem to be directed almost exclusively at the elimination of the meter-reader by employing systems for automatically transmitting information from the electro-mechanical meter to an electronic data collecting medium. While saving manpower, any such system appreciably increases capital costs and does nothing to minimize the high maintenance expense which the complex electro-mechanical meters inherently involve.